1. Field of the Invention
The present invention relates to fiber optic bundles for transporting images, and especially to such fiber optic bundles which have a high image resolution.
2. Discussion of Related Art
With the advent of fiber optics technology, the opportunity of transporting optical images in fiber bundles became available. Such bundles conventionally have a point-to-point relation between the input and the output of the fiber bundle. Such bundles are called "coherent bundles" and are in widespread use, e.g. in fiberscopes, etc.
There are, however, certain shortcomings which need to be overcome. These relate to the size of the input and output area of the bundle and the diameter of the individual fibers used in the bundle.
Since in a coherent bundle, the location of each fiber has to be identical in the input and output plane, it becomes more and more difficult to keep track of each fiber because as the image area becomes larger the total number of fibers is increased.
Further, in any fiber bundle, the diameter of the individual fibers determines the resolving power of the system. The smaller the diameter, the better the resolving power. The maximum acceptable diameter depends on the application. However, at the same time, the smaller the diameter of the optic fiber, the weaker the fiber. If the fiber becomes too small, it is difficult if not impossible to handle without damage.
The resolution of an image can be expressed in line pairs/mm. For example, if a resolution of 100 line pairs/mm is needed, and if at least 4 fibers were required per line pair, then a fiber diameter of 1/400 mm=2.5 micrometers would be required. A resolution of 100 line pairs/mm is achievable with photographic film. Standard fibers are 50 micrometers; therefore, they produce a resolution of about 5 line pairs/mm. A 50 micrometer fiber is barely visible with the naked eye. Photographic emulsions using this size grain would not be considered acceptable. In principle, glass fibers could be drawn out to 2.5 micrometer diameter, but the mechanical strength would be so low that handling of such a bundle would be very difficult, at the least.
If a fiber optic bundle is to compete in resolution with a 35 mm camera, 36.times.400=14400 fibers per image line are required for a total of 24.times.400=9600 image columns or 1.4.times.10.sup.8 fibers altogether. If a fiber optic bundle is to compete in resolution with a large sheet film, e.g., chest x-ray (maybe 10.times.10 in.), on the order of 10.sup.12 fibers are required. These are, of course, fantastic numbers, but they illustrate the basic problem.
In view of the foregoing problems, it can be seen that it is not always feasible to compete with a 35 mm camera. However, several scenarios exist where fiber optic bundles can replace cameras. For example, if pictures are to be taken in high radiation environments, e.g. inside a throughport of a nuclear reactor, photographic recording is impossible. One could transport the image with relay lenses to a low radiation area and then take the picture photographically, or one can use a fiber optics bundle for transportation of the image. Fibers which are radiation hardened are available for such an application. Therefore, in this case one indeed competes with photographic recording. The fiber optics cable can be snaked around corners, while the relay lenses need a clear aperture, giving range to shielding problems. Therefore, a fiber optic bundle would indeed be preferable if it could deliver the required resolving power. The same argument is true for any other periscope arrangement, e.g., for a submarine. Here, also, it would be advantageous to replace the relay lenses by a fiber optics bundle; however, the resolving power cannot be compromised.
Any application where observation of inaccessible locations are attempted, the fiber optics bundle is preferable. If the location is so inaccessible that an optical relay is not possible, one does not compete with the photographic camera anymore, therefore, any resolving power is welcome. If a relay is possible, one has to be able to offer a resolving power comparable to the photographic camera. Thus, it can be seen that a need has developed for a fiber optic device capable of transporting high resolution images.
Various fiber optic bundles have been suggested. For example, Wilcox (U.S. Pat. N0. 3,461,223) uses an ordered fiber array which is electronically scanned. It is intended for color television images. The actual image transmission is accomplished by air (UHF) or electrical cable. The fiber array is a decoding and encoding means only, not a transmission means.
Schackert (U.S. Pat. No. 3,512,861) uses a single row or a limited number of rows of ordered fibers which are mechanically moved to achieve scanning. Image transmission is through one row (or few rows) of ordered fibers.
McIntyre (U.S. Pat. No. 3,652,855) uses an ordered fiber optics bundle connected to a limited number (70) of photomultiplier tubes. An elaborate coding scheme between the input fibers and the photomultiplier tubes, together with a coincidence circuitry, allows obtaining of more than 70 pixels. The fiber bundle used is short and ordered. The coding scheme is specified in greater detail in a successive patent (McIntyre, U.S. Pat. No. 4,379,967).
Schaefer (U.S. Pat. No. 4,052,705) describes actually a memory device for computers. One could construe that it constitutes the transport of images. However, the bundles (mostly one rowers) are strictly ordered and short. They are used to form various types of gates (and, or, not, etc.).
The patent to DeBie (U.S. Pat. No. 4,570,063) also a discloses a one row device which is meant for scanning a document.
There is no known structure suitable for transporting high resolution images through fiber optics.